1. Field of the Invention
The present invention relates to a belt type continuously variable transmission and a control device for a belt type continuously variable transmission.
2. Description of Related Art
A belt type continuously variable transmission includes a primary sheave and a secondary sheave, each having a groove with a variable width. The primary sheave is mounted on a primary shaft to which an output from a power source such as an engine is input, and the secondary sheave is mounted on a secondary shaft from which an output for a driving wheel is output. In general, the primary and secondary sheaves are each made up of a fixed flange and a movable flange defining a V-groove therebetween. Each movable flange is movable in an axial direction of the primary or secondary shaft. A V-belt is wound around both sheaves. The gear ratio of the belt type continuously variable transmission is varied by moving the movable flange to vary the groove widths of both sheaves.
Some V-belt type continuously variable transmissions include a control device for controlling an actuator for moving the movable flange of the primary sheave. The control device calculates a target gear ratio based on vehicle information such as throttle opening degree and vehicle speed, for example. In order to move the movable flange to a target position, the control device sends a control signal to the actuator to control the position of the movable flange. Such V-belt type automatic continuously variable transmissions are applied, for example, to vehicles such as motorcycles (see JP-B-3043061, for example).
JP-A-Hei 7-158706 includes a position sensor for detecting the position of the movable flange. JP-A-Hei 7-158706 discloses that factors such as variations in output characteristics of the position sensor, changes over time in the characteristics, and elongation and wear of the V-belt impair the consistency of the correlation between the value detected by the position sensor and the position of the movable flange. JP-A-Hei 7-158706 further discloses self-correction of data of the impaired consistency of the correlation based on the ratio between the speeds of the primary and secondary sheaves.
The transmission control method disclosed in JP-A-Hei 7-158706, however, requires complicated processes such as determination of the presence/absence of a slip of the belt, calculation of an input torque, and so forth.
Various control methods with less variations in the correlation described above among vehicles and with less influence of such variations on control have been conceived. For example, the correlation can be corrected by performing measurement over the entire movable range of the movable flange. Such correction needs to be made for each vehicle, and cannot treat changes over time. Another method calculates an actual gear ratio and uses the calculation results as feedback in order to control the gear ratio of the belt type continuously variable transmission. In this method, however, the gearshift response of the vehicle is worsened. Therefore, it is desirable to improve control accuracy in an easier way.